“General Kelly, there is no ‘compromise’ regarding slavery. Ever,” Clinton tweeted. “And, the Constitution’s original 3/5ths Compromise was an abomination.”

Clinton was referring to a 1787 agreement reached between Northern and Southern states that, for purposes of determining representation in Congress, counted each free person in the South as one person — and each slave as three-fifths of a person.

More then 600,000 Americans are estimated to have died in the Civil War.